Systems and methods for converting gases to liquids

ABSTRACT

A system and method of converting natural gases to liquids is provided. The system includes a catalytic partial oxidation (CPO) system with natural gas, air and steam input, a Fischer-Tropsch (F-T) system taking syngas from the CPO system, and supplying product gases to a power engine (PE), after separation of the product liquids. An F-T steam output line is in fluid communication with the CPO-steam input line. The energy output from the PE is supplied to the compressors and condensers, to provide self-sustainability in energy, for the gas-to-liquid separation system.

BACKGROUND

The invention relates generally to systems and methods for convertinggases to liquids. More particularly, the invention relates to convertingnatural gases to liquids using catalytic partial oxidation.

Natural gas or other gaseous hydrocarbons are normally converted into aliquid form, such as longer-chain hydrocarbons, using a large scalegas-to-liquid process. Methane-rich gases are converted into liquidfuels using syngas as an intermediate, as in a Fischer Tropsch (F-T)process.

An F-T process is a set of chemical reactions that convert a mixture ofhydrogen (H₂) and carbon monoxide (CO) into liquid hydrocarbons. Themixture of H₂ and CO may be obtained by subjecting natural gas topartial oxidation. Catalytic partial oxidation (CPO) is one of suchpartial oxidation processes that oxidize the natural gas in the presenceof oxygen, over a catalyst. Treatment of natural gas in a CPO processnormally yields H₂, CO, carbon dioxide (CO₂), and water. The H₂ and COcan be used in the subsequent F-T process.

In the CPO process, a pure oxygen input is typically used to obtain acleaner (without nitrogen dilution) output, so as to obtain highercarbon conversion efficiency from CO to hydrocarbons in an F-T reaction.However, producing oxygen by separating oxygen from air typicallyrequires an air separation unit (ASU), which further requires an inputof energy. The additional energy requirement for producing oxygen forthe CPO process, and the significant capital investment for producing amixture of H₂ and CO (a mixture commonly referred to as syngas) for theF-T process increases the cost of producing liquids from the gases.

As noted above, the syngas is chemically reacted in the F-T reactionover a catalyst to produce liquid hydrocarbons and other byproducts.However, the H₂-to-CO ratio obtained from a typical CPO process may notbe the optimal ratio for carrying out the F-T reaction. Normally, theH₂-to-CO ratio obtained from a partial oxidation reaction may be lowerthan what is required for the F-T reaction. The ratio of H₂-to-CO may beadjusted before entering the F-T system, by using a water gas shiftreaction or alternatively, carrying, out steam methane reforming (SMR),instead of CPO. The water gas shift reaction involves reaction of waterwith CO to produce H₂, and CO₂, hence increasing the H₂-to-CO ratio. Theexcess carbon dioxide may be removed before the gases enter the F-Tsystem.

The SMR reaction is an alternative hod to produce syngas with a higherH₂-to-CO ratio (syngas ratio). In this process, methane is reacted withwater to produce H₂ and CO, with a syngas ratio of about 3.0. This ratiois higher than is required by the F-T reaction. Further, the SMRreaction is an endothermic reaction. Therefore, a portion of the naturalgas is usually combusted, to provide energy for the SMR reaction. Sincea portion of the feed is combusted instead of being used to generate H₂and CO, the overall conversion efficiency of the SMR reaction isundesirably reduced. In general, the requirement for external heating,and the higher syngas ratio, are two primary drawbacks in using SMR forgas-to-liquid conversion.

Formation of liquid hydrocarbons such as alkanes in the F-T process isdesirable. However, methane formation from the F-T reaction is generallynot desirable. The F-T process is generally operated in the temperaturerange of about 190° C.-350° C. Higher temperatures lead to fasterreactions and higher conversion rates. However, the higher temperaturesalso favor methane production.

Another method of increasing F-T reaction rates and conversion is byincreasing the pressure within the F-T system. A typical method forincreasing the pressure within an F-T system includes compressing thesyngas before entering the F-T system. However, pressurizing the syngasbefore entering the F-T system requires more energy input to the overallsystem, thereby increasing the cost of gas-to-liquid conversion.

Therefore, there is a need to reduce the energy input to the overallprocess of converting gases to liquids. A process that requires noenergy input (or a greatly-reduced energy input) for the ASU, for syngasproducing reaction, and/or for syngas compression, may decrease theoverall cost of producing hydrocarbon liquids from natural gas.Furthermore, eliminating the step of balancing the H₂-to-CO ratio maybenefit the overall process.

BRIEF DESCRIPTION

Briefly, in one embodiment, a system is provided. The system includes acatalytic partial oxidation (CPO) system, a Fischer-Tropsch (F-T)system, and a power engine (PE) system. The CPO system itself usuallyincludes a CPO-input line and a CPO-output line. The CPO-input line isin fluid communication with a CPO-natural gas input line, CPO-air inputline, and a CPO-steam input line. The F-T system usually includes anF-T-reactor input line, an F-T-reactor output line, an F-T water inputline, and an F-T steam output line. The F-T-reactor input line isusually in fluid communication with the CPO-output line and anF-T-syngas compressor. The F-T-reactor output line is usually in fluidcommunication with an F-T-liquid output line, and an F-T-gas outputline. The F-T steam output line is in fluid communication with theCPO-steam input line. The PE system includes a PE, a PE input line, anda PE-output line. The PE-input line is in fluid communication with theF-T-gas output line, and the PE is configured to provide energy to theF-T-syngas compressor.

In one embodiment, a method is provided. The method includes the stepsof feeding natural gas, air, and steam to a CPO system as a CPO input,starting the CPO initial reaction by providing external heat to the CPOsystem, carrying out a CPO reaction to produce a CPO output comprisingsyngas and nitrogen, feeding at least a part of the CPO output as aFischer-Tropsch (F-T) reactor input to an F-T system; feeding water asan input to the F-T system; carrying out an F-T reaction in the F-Tsystem to produce an F-T steam output, and an F-T reactor outputcomprising F-T liquids, and F-T gases; feeding the F-T gases as an inputto a power engine (PE); and supplying at least a portion of the F-Tsteam as a feed to the CPO system.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings, inwhich like characters represent like parts throughout the drawings,wherein:

FIG. 1 illustrates a system for converting gases to liquids, accordingto an embodiment of the invention; and

FIG. 2 illustrates a Fischer-Tropsch (F-T) reactor, according to anembodiment of the invention.

DETAILED DESCRIPTION

One or more specific embodiments of the present invention will bedescribed below. In an effort to provide a concise description of theseembodiments, all features of an actual implementation may not bedescribed in the specification. It should be appreciated that in thedevelopment of any such actual implementation, as in any engineering ordesign project, numerous implementation-specific decisions must be madeto achieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill, having the benefit of thisdisclosure.

In the following specification and the claims, the singular forms and“the” include plural referents, unless the context dictates otherwise.

Embodiments of the present invention provide a system and method forconversion of gases to liquids. Particularly the system converts naturalgases to hydrocarbon liquids. The process of conversion as per theembodiments of this invention includes syngas (a mixture of H₂ and CO)production from a catalytic partial oxidation (CPO) reaction, andprocessing the syngas in a Fisher Tropsch (F-T) system to producehydrocarbon liquids. Further, the F-T system converts water to steam,using the F-T reaction heat. The steam may be fed to the CPO system. Thehydrocarbon liquids may be separated from the F-T system products, andthe gases of the F-T system product may be fed into a power engine (PE),to produce energy.

FIG. 1 schematically represents a system as per one embodiment of thepresent invention. In FIG. 1, the system 10 for converting gases toliquids includes (as indicated by the “boxed lines”) a CPO system 20, anF-T system 60, and a power engine (PE) 100. The CPO system 20 includes aCPO reactor 22, a CPO-input line 24 and a CPO-output line 26. TheCPO-input line is in fluid communication with (as that term is definedbelow) a CPO-natural gas input line 28, a CPO-air input line 30, and aCPO-steam input line 32.

The F-T system 60 includes an F-T reactor 62, an F-T-reactor input line64, an F-T-reactor output line 66, an F-T water input line 68, and anF-T steam output line 70. The F-T-reactor input line 64 is in fluidcommunication with an F-T-syngas compressor 72. The CPO-output line 26of the CPO system 20 is in fluid communication with the F-T-syngascompressor 72 and the F-T-reactor input line 64. The F-T-reactor outputline 66 is in fluid communication with an F-T product separator 74,F-T-liquid output line 76, and an F-T-gas output line 80. The F-T steamoutput line 70 is in fluid communication with the CPO-steam input line32.

The PE (power engine) system 100 includes a PE 106, a PE input line 102,and a PE-output line 104. The PE-input line 102 is in fluidcommunication with the F-T-gas output line 80. The PE 106 is incommunication with the F-T-syngas compressor 72, e.g., by an energysupply conduit (not specifically depicted in the drawings) that providesenergy to the F-T-syngas compressor 72, to compress the syngas.

FIG. 2 schematically represents the operation of an F-T system. The F-Treactor 62 of F-T system 60 (FIG. 1) includes an inner, reaction region82, and an outer region 84. The F-T reactor input line 64 and the F-Treactor output line 66 are in fluid communication with the inner region82, of the F-T reactor 62. The F-T water input line 68 and the F-T steamoutput line 70 are in fluid communication with the outer region 84 ofthe F-T reactor. As noted earlier, the F-T system 60 is a schematicrepresentation, and may include multiple channels in the reactor region82 and/or the outer region 84.

In one embodiment of the invention, the CPO system 20 (FIG. 1) furtherincludes three heat exchangers. 34, 36, and 38. The heat exchanger 34,for example, can be in communication with the CPO-natural gas input line28, and the CPO-output line 26. The heat exchanger 36 is incommunication with the CPO-air input line 30, and the CPO-output line26. The heat exchanger 38 is in communication with the CPO-steam inputline 32, and the CPO-output line 26. The heat exchangers 34, 36, and 38may exchange their places, depending on the design of the CPO system 20.In some embodiments, The CPO system 20 may further include a natural gascompressor 46, and an air compressor 48. The F-T system 60 may furtherinclude a syngas condenser 86, and a water purifier 88. The natural gascompressor 46 may deliver compressed natural gas to the CPO reactor 22,to further enhance efficiency of the CPO reactor. Similarly, the aircompressor 48 may compress air and deliver the compressed air as aninput to the CPO reactor 22. The syngas condenser 86 may condense themoisture from the CPO-output, and supply at least a part of the syngasto the F-T reactor 62. The natural gas compressor 46, and air compressor48 may further be in communication with the PE 106 (not shown infigures).

As used herein, the term “in fluid communication with” implies thatdifferent systems or system units are connected to each other with somefluid input or fluid output. This communication may be direct orindirect, i.e., passing through some intervening units or sections. Forexample, in one embodiment, as shown in FIG. 1, the F-T steam outputline 70 is in fluid communication with the CPO-steam input line 32directly without having any intervening units, while the CPO-output line26 is in fluid communication with the F-T reactor input line 64 throughthe intervening units such as F-T syngas compressor 72, and the syngascondenser 86. The “CPO-output line 26 is in fluid communication with theF-T reactor input line 64” means that the F-T reactor input line 64receives the syngas from the CPO-output line 26.

As used herein, the term “in communication with” implies that differentsystems or system units are connected to each other with some input oroutput. This communication may be direct or indirect, i.e., passingthrough some intervening units or sections. Further, the “input” and“output” as used herein includes the passage of any fluids, heat, orenergy. For example, the “PE 106 is in communication with the F-T-syngascompressor 72” implies that the PE 106 supplies energy for thecompression of syngas in the syngas compressor 72. Similarly, as shownin FIG. 2, the F-T reactor inner region 82 and the F-T reactor outerregion 84 may be in communication with each other through heat transfer.

In one embodiment, a method of converting natural gases to liquids usingthe system 10 is provided. The method includes providing the naturalgas, along with air and steam, to the CPO reactor 22.

The air as used herein may be natural air, or an oxygen-enriched air. Inone embodiment, the percentage of oxygen in the air is less than about25 volume %. In one embodiment, the percentage of oxygen in the air isin the range from about 18 volume % to about 22 volume %. In someembodiments, a ratio of oxygen to carbon in a combined input of air andnatural gas may be controlled to be in a range from about 0.5 to about1.5. In one embodiment, the ratio of oxygen to carbon is in a range fromabout 1 to about 1.4. In some embodiments, this range of about 1 toabout 1.4 is preferred, because it appears to provide enhancedefficiency.

The use of air in systems like those described herein is in surprisingcontrast with conventional systems that include CPO reactors, e.g., forconverting gases to liquids. In the conventional systems, usage of airwas often thought to be undesirable. One reason relates to the presenceof the other components in air, such as nitrogen, that dilute thedesired syngas resulting from the CPO reaction. However, in embodimentsof the present invention, the use of air is acceptable, and in someinstances may be a very desirable attribute, as described herein.

The diluted syngas can be converted to a mixture of light and heavyhydrocarbons mixed with nitrogen. The liquids can be separated, leavingthe light hydrocarbons and nitrogen in the gas phase. As shown in FIG.1, for example, the gases from the F-T gas out line 80 may be used as aninput to the PE system 100, to produce energy. In one embodiment, asshown in FIG. 1, the separation of the byproduct light hydrocarbons andnitrogen) gases from liquids is carried out after the F-T process issubstantially complete, in the separator 74. The separator 74 is anapparatus configured to cool down the product that exits the F-Treactor. The reduced temperature causes the heavier hydrocarbons tocondense and separate out, leaving the nitrogen and light hydrocarbons,such as methane, in the gas phase. These gases from the separator may befed to the PE system 100 as an input.

As described above, syngas produced from some of the conventionalCPO-based processes is characterized by a lower syngas ratio thanrequired for the F-T process. In one embodiment of the presentinvention, the syngas ratio (H₂-to-CO ratio) for the CPO reactor 22should be about 2, e.g., in those situations in which the F-T system 60employs a cobalt-catalyst. As used herein, a ratio of “about 2” refersto a value greater than 1.95 and less than 2.25. Different techniquesmay be used to adjust the syngas ratio to a desired value. Usually, thesyngas ratio may be controlled by controlling two main parameters, suchas the oxygen-to-carbon ratio, and the steam-to-carbon ratio. Dependingon the desired syngas ratio for the F-T reactor, the amount of air(which includes oxygen), steam, and natural gas may be controlledrelative to each other, to achieve the desired oxygen-to-carbon ratioand steam-to-carbon ratios. For example, and with reference to FIG. 1,the syngas ratio of the CPO reactor 22, may be adjusted by directingsteam to the CPO reactor 22, through the CPO-steam input line 32. Thesteam can serve as an additional source of hydrogen to adjust the syngasratio. In one embodiment, the CPO input has a steam-to-carbon ratio (inthe CPO-input line 24) in a range from about 0.3 to about 1.5. In afurther embodiment, the steam-to-carbon ratio in the CPO-input line 24is in a range from about 0.5 to about 1. (In some preferred embodiments,the desired ratio of hydrogen to carbon monoxide in the syngas is in therange of about 1 to about 3).

In one embodiment, at least a portion of the gaseous output of the CPOreactor 22 is supplied to the F-T system 60. System 60 provides thereaction of the gases over the catalyst, to covert the syngas intohydrocarbon liquids. The F-T system may use different catalysts, such asthose based on cobalt, iron, or ruthenium. In one embodiment, thecatalyst used by the F-T system is a cobalt catalyst. The catalyticreaction occurring in the F-T reactor inner region 82 (FIG. 2) isexothermic, and hence gives off heat. Water, which may be purified in apurifier 88 (FIG. 1, and depicted simply with the arrow aimed into 88),is supplied to the outer region 84 of the F-T reactor 62 (FIG. 2). Theheat output of the F-T process in the inner region 82 heats up the waterin the outer region 84, and converts the water into steam. In oneembodiment, the steam output from the F-T system 60 is passed as thesteam input to the CPO reactor 22. In one embodiment, the steam exitingthe F-T reactor 62 is at a temperature in a range from about 100° C. toabout 350° C.

It is usually desirable to heat the input constituents to the CPOreactor 22 beforehand, in order to increase the overall efficiency ofthe system. However, the CPO reaction itself is exothermic and hence,liberates heat. The liberated heat is carried by the CPO output, passingthrough the GPO-output line to the F-T input line 64. In one embodiment,the F-T reactor requires a syngas input at a temperature lower than theCPO-output. Therefore, in one embodiment, the CPO system 20 includes oneor more heat exchangers 34, 36, and 38 (described briefly above),configured to take out heat from the CPO output. For example, as shownin FIG. 1, at heat exchanger 34, the CPO output exchanges heat with thenatural gas input to the CPO reactor 22, and thereby provides heatednatural gas input to the CPO-input line 24, via line 40. Similarly, theair input that is heated up by the heat exchange with the CPO output atthe heat exchanger 36, is directed to the CPO-input line 24, through theinput line. Furthermore, the steam input is heated as it is transportedto the CPO-input line 24, through input line 44, by the heat exchangewith the CPO output at the heat exchanger 38. The CPO output, afterpassing through the heat exchangers with the CPO inputs, becomes cooler.The cooled CPO output (including syngas) may be further cooled in acondenser 86 (mentioned above), to remove moisture. The gas can then becompressed in the syngas compressor 74, to a higher pressure relative tothe CPO output pressure. In one embodiment, the cooled CPO output iscompressed to a pressure in a range from about 5 atmospheres to about 60atmospheres.

In the F-T reactor 62, the syngas reacts in the presence of a catalyst,and yields an F-T reactor product that is directed into the F-T reactoroutput line 66. In one embodiment, the product is a mixture of gases andliquids. The gases are separated out from the product liquids in aseparator 74. The separator 74 may be any kind of gas-liquid separationdevice. After the separation, the product-liquid is directed through theF-T liquid output line 76, and the product gases are directed into theF-T gas output line 80. Since air, rather than oxygen, is supplied tothe CPO system 22 in the oxidation reaction, the product gases from theF-T reactor will include nitrogen among other gaseous products. In oneembodiment, the nitrogen content in the F-T reactor product gases isgreater than about 20 volume %.

As described earlier, in one embodiment, the gaseous products obtainedafter the separation from the F-T reactor liquid products are directedas an input to the PE 106, through a PE input line 102. The power engine106 may be any kind of power production unit that may use the gaseousinputs as a fuel. In one embodiment, the PE 106 is a reciprocatingengine, such as a Jenbacher engine. The PE 106 typically produceselectrical energy. The electrical energy of the PE 106 may be taken outthrough the PE output line 104, and supplied for a suitable energy need.

In an embodiment, the CPO reactions are exothermic reactions. However,it is usually desirable that some external heat may be provided toinitiate the CPO process. In one embodiment, the heat is provided byheating the initial input gases. The CPO reaction may be initiated bypassing the input gases over the catalyst in the CPO reactor 22, for theinitial reaction at a temperature less than about 400° C. As usedherein, the “initial reaction” is the first reaction producing heatedCPO output. In one embodiment, the desired temperature within the CPOreactor 22 is in the range from about 275° C. to about 325° C. Once thereaction begins in the CPO reactor 22 (i.e., after an initial start-upperiod), the CPO reaction may be carried out in the absence of anyexternal heat input, and this can be a desirable system and processadvantage.

For the CPO reaction, the natural gas and air may be supplied atatmospheric pressure, or supplied in a compressed form. The natural gascompressor 46 and air compressor 48 require energy to operate.Similarly, the syngas compressor 72 requires energy to compress thesyngas. The syngas may be compressed significantly, e.g., to at leastabout 5 atmospheres, to increase the F-T reaction pressure. An F-Tprocess conducted at high pressures produces desirable long chainalkanes C₅-C₂₀ and higher carbon number hydrocarbons) as the primary F-Tliquid product.

In some embodiments, the integration of PE 106 with the CPO system 20and the F-T system 60 provides the energy input needed for thecompressors and other ancillary equipment, such as instrumentation andcontrol equipment. Therefore, in some embodiments, the system 10 for theconversion of gases to liquids becomes self-sustainable, without anyfurther external energy input, i.e., other than the heat required forthe initial CPO reaction.

It should be apparent from the discussion above that another embodimentof the invention is directed to a system that comprises:

a) a catalytic partial oxidation (CPO) system configured to convertnatural gas, air, and steam, to syngas, in a CPO reaction;

b) a Fischer-Tropsch (F-T) system in communication with the CPO system,and configured to receive at least a portion of the syngas from the CPOsystem, and to produce a steam product, and F-T liquids and gases;

wherein the F-T system comprises a compressor for compressing thesyngas; and also comprises a conduit (e.g., any type of connection orpassageway), configured to direct at least a portion of the steamproduct to the CPO system for carrying out the CPO reaction; and

c) a power engine, in communication with the F-T system and at leastpartially fueled by the F-T liquids or gases, and configured to supplyenergy to the syngas compressor.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

The invention claimed is:
 1. A method comprising: a. providing naturalgas, air, and steam to a catalytic partial oxidation (CPO) system as aCPO input; b. carrying out a CPO reaction to produce a CPO outputcomprising syngas and nitrogen; c. feeding at least a portion of the CPOoutput as a Fischer-Tropch (F-T) reactor input, to an F-T system; d.feeding water as an input to an outer region of the F-T system; e.carrying out an F-T reaction in the F-T system to produce an F-T steamproduct, and an F-T reactor output comprising F-T liquids and F-T gases;f. feeding the F-T gases as an input to a power engine (PE); and g.supplying at least a portion of the F-T steam as a feed-input to the CPOsystem.
 2. The method of claim 1, wherein a temperature of the CPOsystem during the initial CPO reaction is less than about 400° C.
 3. Themethod of claim 1, wherein after an initial start-up period, the CPOreaction is carried out in the absence of any external heat input. 4.The method of claim 1, wherein the CPO input has an oxygen to carbonratio in a range from about 0.5 to about 1.5.
 5. The method of claim 1,wherein the CPO input has a steam to carbon ratio in the range fromabout 0.3 to about 1.5.
 6. The method of claim 1, wherein the syngascomprises hydrogen and carbon monoxide, and the ratio of hydrogen tocarbon monoxide is in the range of about 1 to about
 3. 7. The method ofclaim 1, further comprising the step of compressing the F-T input, priorto feeding the F-T input to the F-T reactor.
 8. The method of claim 1,wherein the PE supplies power for the compression of the F-T input. 9.The method of claim 1, wherein the F-T-reactor input comprises nitrogenin a range from about 20 volume % to about 50 volume %.
 10. The methodof claim 1, wherein the CPO reaction is carried out at a temperature ina range from about 700° C. to about 1000° C.
 11. The method of claim 1,wherein the CPO input exchanges heat with the CPO output to produce acooled CPO output stream.